1. Field of the Invention
The embodiments herein generally relate to microelectromechanical systems (MEMS) technology and, more particularly, to electromagnetic MEMS devices.
2. Description of the Related Art
There are many military uses for seismometers and accelerometers. For example, accelerometers have been used for years for safe and arming (S&A) in the area of fuzing. Tests show that seismometers are one of the best sensors for detecting personnel. Accelerometers can be used for pyroshock, that is the transient response of a structure to loading induced by the ignition of pyrotechnic (explosive or propellant activated) devices. Generally, MEMS accelerometers are sensors that use a variety of approaches that include measuring capacitance changes, tunneling, and using piezoelectric, ferroelectric and optical materials. Accelerometers have also been used in controlling and monitoring military and aerospace systems. This last application includes smart weapon systems (e.g., direct and indirect fire; and aviation-launched and ship-launched missiles, rockets, projectiles, and submunitions). However, there is a need for low cost, small, sensitive accelerometers.
There are accelerometers and seismometers that make use of magnetic components. One of the most standard seismometers measures the voltage induced when a permanent magnetic moves inside a coil. The magnetostriction of amorphous materials bonded onto cantilevers has been used to make accelerometers. These solutions utilize Fe67Co18B14Si, which is a material with a saturation magnetostriction of 35 ppm. Other solutions use a magnetic spring interacting with a permanent magnet to make a horizontal component seismometer. Moreover, high sensitivity accelerometers can be fabricated using superconductivity. Unfortunately, these devices tend to be rather expensive to fabricate.
Some seismometers can detect 10−5 G's of force. Furthermore, some seismometers have an operating range of DC to 100 Hz. Generally, one problem with these conventional seismometers is that they are rather large in size. Moreover, it is generally difficult to couple them to the ground or other surfaces. For example, one may have to mechanically couple them to the ground by inserting spikes into the ground. Accordingly, using these massive seismometers in a building may be especially difficult.
Some MEMS seismometers use changes in capacitance to measure the acceleration. The sensitivity of most conventional accelerometers is approximately 0.001 g. Most conventional MEMS accelerators use capacitive transduction because they consume little power, have good temperature stability, and are energy efficient.
The MEMS flux concentrator is a device that mitigates the effect of 1/f noise in a sensor by shifting the operating frequency to higher frequencies where 1/f noise is much less important. It does this by placing the flux concentrators on MEMS structures. The motion of the flux concentrators modulates the field seen by the sensor. Typically, SOI wafers are used in the fabrication of the device, wherein the device typically consists of a MEMS structure with MEMS flaps covered by a soft magnetic material (permalloy), a comb drive, and a spin valve magnetic sensor. The device can improve the sensitivity of magnetoresistive sensors by two orders of magnitude.
The conventional accelerometers and seismometers generally have not been sufficiently small or sensitive to be properly utilized in MEMS applications. Accordingly, there remains a need for a new, small, low cost accelerometer or seismometer.